Many existing vehicle and aircraft designs incorporate numerous separate wires connecting the various electrical and electronic devices in the vehicle system. Generally, these multiple wires are tied together into a wiring harness, which may often contain dozens of wires.
Aircraft electronics systems have particularly large numbers of wires interconnecting the aircraft electronic devices. Even a transponder may have thirteen wires connecting to an altitude encoder, and another fifteen connecting to a remote control head, for a total of almost 30 wires. Other aircraft electronics devices often connect to many more wires than this.
Vehicle and aircraft wiring systems are generally subjected to vibration and corrosive atmospheres. As a result, they often develop flawed connections. Flawed connections may be constant, or may be intermittent.
Constant faults are fairly easy to identify and fix, because the fault remains present while a technician traces the circuitry to locate the fault. Intermittent faults are hard to diagnose and repair because they occur with some randomness, and then may occur only under conditions of system stress. Even when an intermittent fault occurs while a system is under stress at a repair facility, the fault can often be diagnosed only when the fault occurs while an instrument is monitoring the faulty connection because intermittent faults often are present in the system for only a short time, too short a time to allow tracing of circuitry to locate the problem.
A method for diagnosing intermittent faults in wiring harnesses or vehicle electronic systems including wiring harnesses is for a technician to attach a latching continuity tester to each wire of the harness. The vehicle, or harness, is then stressed by vibration, heating, or cooling such that any intermittently flawed connection in the wiring will, at least momentarily, fail. The latching continuity tester detects the momentary fault, and latches the identity of the momentary fault. The tester then provides the identity of the failed connection to the technician.
A similar technique can be employed on a printed circuit card, or complete electronic module. A multichannel continuity tester is attached to the card or module, the module is then stressed. Any momentary failures are latched by the tester.
The data input and output terminations of many common electronics modules have signal levels ranging from 0 to 5 volts. Most of these modules have a TTL or CMOS integrated circuit connected to each of these signal terminations. TTL and CMOS integrated circuits generally have a parasitic diode connection between the signal termination and ground. This diode becomes forward biased when the signal termination is driven negative with respect to ground, clamping the signal termination to a level of typically −0.7 volt. This parasitic diode is commonly utilized in testing continuity of devices during the integrated circuit manufacturing process.
Signature testing has become popular for testing electronic apparatus. A signature tester monitors the performance of a circuit subjected to a repeatable stimulus, and computes a pattern or number, or signature, corresponding in some way to the response of the circuit. For very simple circuits, the signature may comprise the entire response of the circuit. For larger circuits having a more complex response, the signature bit pattern is generally substantially smaller than the entire circuit response. The signature is computed through a hashing algorithm such that all good circuits have the same signature, but most, if not all, defective circuits have different signatures. Once computed, the signature derived from the circuit under test is compared with the signature of a known good circuit. One example of signature testing using a neural network is described in U.S. Pat. No. 5,744,967, incorporated herein by reference.